Table Level in Navision 2009 R2

Your code added to CAmount - OnValidate is not going to work. The OnValidate trigger can be activated (called) either from code, by calling VALIDATE(CAmount), or when CAmount is exposed on the Page and is made editable, so the user can change value in it. CAmount - OnValidate trigger is never called be the system when data in other tables changes in a way which would change the content in CAmount

Take the code out of CValidate trigger, and move it to Amount - OnValidate. also add CALCFIELDS(CAmount) call before using it in your calculation:

Amount - OnValidate()
CALCFIELDS(CAmount)
"Balance Amount" := Amount - CAmount;

Your balance will only be updated if you validate/change Amount field. Changes in CAmount will not be automatically reflected in your "Balance Amount".